How about a large scale version of the open sided excursion car.  The trucks,frame and roof come from existing passenger car patterns and the seats and some other patterns are available from the open sided trolley.

For those of you wanting a narrow gauge diesel, look around for one of the Spectrum 45 tonners.  They are large but remember, a lot of NG diesels were just standard models on NG trucks.  As I recall, they were eventually dumped on the market due to slow sales.

I agree, a C-16 would be nice.